According to one source, it can take up to 2 or more years before blooming. The seeds should be sown around the fall equinox.

-Nick

I made the mistake of not deadheading my 'Purple Sensation' Alliums. I now have what seems like millions of the thing. They take a few years to bloom, but I believe they germinate the following spring. I don't bother to harvest the seeds when I want more--I just bang the seed heads together in the desired area and look for chive-like seedlings the following year.

I planted purple sensation allium last fall (my first year with any allium) and they bloomed and looked beautiful this spring. I dug up a few bulbs after the flowers died and saw that the bulbs multiplied. Am I correct in assuming that each clove will produce another allium plant if replanted this fall? If the bulbs are not separated, will a number of allium grow "together?"
Thanks,
foxydigitalis

I don't know the answer to your questions, but they interested me,too. So I'm hoping someone with the knowledge will see this and answer us. Thank you.

I am not familiar with purple sensation, but other alliums grown for flowers can be left in the ground. You do not need to devide the bulbs. Next year the clump will be bigger.

Alliums can grow quite close together and still flower well. If you see flower production go down in a few years, then divide them.